1. Field of the Invention
The present invention generally relates to a comb filter for separating a luminance signal and a chrominance signal from a composite color video signal to output the separated signals and, more particularly, to a video apparatus such as a color television receiver or a VTR that uses this comb filter.
2. Description of Related Art
Ordinarily, for a television receiver to obtain an output image from a received composite color video signal, a luminance signal and a chrominance signal must be separated from the composite color video signal. As for a VTR, an input composite color video signal is separated into a luminance signal and a chrominance signal and the luminance signal is frequency modulated and the chrominance signal is frequency converted to the low frequency side of the FM luminance signal for recording.
For a means to separate the luminance signal and the chrominance signal from the composite color video signal as described above, a comb filter has come to be frequently used that uses the correlation of the screen vertical direction of color video signal and the inversion of the phase of chrominance subcarrier every period. This is because separation of the luminance signal and the chrominance signal by use of a band filter such as a bandpass filter mixes a high-frequency component of the luminance signal included in the band of the chrominance signal into the chrominance signal to cause an image degradation known as cross color, whereas use of a comb filter prevents this cross color from occurring.
FIG. 8 is a block diagram illustrating a basic circuit constitution of the comb filter.
To be specific, a composite color video signal Sa including a luminance signal Y and a chrominance signal C inputted from an input terminal 11 is supplied to a delay circuit 12 of one horizontal period to be delayed by one horizontal period and the delayed signal is supplied to a first subtracting circuit 13. The composite color video signal Sa from the input terminal 11 is also supplied directly to the first subtracting circuit 13.
When a subtracting operation is performed in this subtracting circuit 13, the luminance signal component is canceled because of the vertical correlation of the video signal since the luminance the signals in adjacent horizontal intervals are almost the same. On the other hand, the phase of the color subcarrier is inverted every horizontal period, so that the color subcarrier components of the adjacent horizontal intervals come in phase in the subtracting circuit 13. As a result, only the chrominance signal component is obtained from the subtracting circuit 13. The chrominance signal component obtained from the subtracting circuit 13 is supplied to a bandpass filter 14 having a pass band of 3.58 MHz±500 kHz (in the case of NTSC signal) for example. From this bandpass filter 14, a chrominance signal C is taken to be sent to an output terminal 17C.
On the other hand, the composite color video signal Sa inputted through the input terminal 11 is supplied to a second subtracting circuit 16 via a delay circuit 15. The delay circuit 15 provides a timing match between the composite color video signal Sa and the chrominance signal C outputted from the bandpass filter 14. This delay circuit 15 provides a delay obtained by summing the delay of one horizontal period by the first delay circuit 12 and the delay provided by the subtracting circuit 13 and the bandpass filter 14.
To the second subtracting circuit 16, the chrominance signal C obtained by the bandpass filter 14 is supplied to be subtracted from the composite color video signal. Therefore, from the second subtracting circuit 16, a luminance signal Y is obtained to be sent to an output terminal 17Y.
Thus, according to the comb filter, the luminance signal Y and the chrominance signal C can be separated from the composite color video signal without causing a cross color.
However, if a composite color video signal in which a change is made from a horizontal scan line representing a uniformly flat screen content to a horizontal scan line representing a signal content in which the phase of chrominance signal is inverted is inputted in the above-mentioned comb filter, the scan line with the phase of chrominance signal inverted has no vertical correlation of the chrominance signal. Therefore, the chrominance component remains on the luminance signal Y at the output terminal 17Y in this scan line, thereby causing a so-called dot crawl.
In the chrominance signal sent to the output derived at the output terminal 17C, the signal level in the scan line concerned drops, thereby deteriorating the vertical resolution of chrominance signal.
To solve this problem, the applicant hereof proposed (in Japanese Non-examined Patent Publication No. Hei 3-70383 for example) a comb filter of adaptive type in which the vertical correlation between adjacent three lines is monitored and, between the lines having the vertical correlation, the luminance signal and the chrominance signal separated by the comb filter are used as outputs and, for the lines having no vertical relation, the luminance signal and the chrominance signal separated by the bandpass filter are used as outputs.
According to the above-mentioned proposition, for the scan lines having no vertical correlation with chrominance signal, the luminance signal and the chrominance signal separated by the bandpass filter are used as output signals to prevent a dot crawl from occurring.
In the above-mentioned adaptive comb filter, the vertical correlation of the video signal between three horizontal scan lines must be monitored. Therefore, as described in the above-mentioned publication, two delay circuits for delaying for delaying a composite color video signal for one horizontal period are used to generate a signal delayed behind the current input signal by one horizontal period and a signal delayed behind the current input signal by two horizontal periods to monitor the vertical correlation between the current input signal and the signal delayed by one horizontal period and the vertical correlation between the signal delayed by one horizontal period and the signal delay by two horizontal periods.
Thus, in the constitution of the ordinary adaptive comb filter, the two delay circuits of one horizontal period are required to delay the composite color video signal, thereby presenting a problem of comparatively increasing the circuit scale. In addition, this constitution must have the circuit for monitoring the vertical correlation between the current input signal and the signal delayed by one horizontal period and the circuit for monitoring the vertical correlation between the signal delay by one horizontal period and the signal delayed by two horizontal periods separately, thereby presenting a problem of complicating the circuit constitution.